1. Technical Field
The present invention relates an ion source having a structure where a cathode is heated by a filament to emit thermal electrons for generating plasma into a plasma generating chamber also serving as an anode. Such an ion source is also referred to as an indirectly heated cathode type ion source.
2. Related Art
This type of related art ion source has a structure where a tubular cathode holder is inserted into a plasma generating chamber with a gap between itself and the plasma generating chamber and a cathode is held at the tip of the cathode holder and a filament to heat the cathode is arranged in the cathode holder (for example, refer to JP-2995388, paragraph 0009, FIG. 6; JP-A-10-134718, paragraph 0009, FIG. 7; U.S. Pat. No. 2004/0061668 A1, paragraph 002, FIG. 1).
In the ion source, the tubular cathode holder is inserted into the plasma generating chamber, and an area where the plasma is generated is made smaller by at least the volume of the cathode holder. This lowers the ionization efficiency of a gas for generating plasma in the plasma generating chamber thus degrading the plasma generation efficiency as well as reduces the plasma volume. Therefore, it is difficult to increase the beam current of ion beams to be extracted from the ion source.
The gap between the cathode holder and the plasma generating chamber serves as an escape route of the gas for generating plasma. This lowers the use efficiency of the gas. The gas for generating plasma generally cost high. A reduced use efficiency of the gas leads to a higher operation cost of the ion source. Leakage of gas may contaminate a structure on the periphery of the plasma generating chamber, which shortens the service life of the ion source.
Further, the cathode wears with the operation time of the ion source. Although a larger axial length of the cathode (or depth of the cathode) is advantageous in terms of the service life of the cathode, and thus, the ion source, it is difficult to provide a long cathode in the related art ion source. A longer cathode results in a larger heat loss caused by emission from the side surface of the cathode, which makes it difficult to heat the cathode. Moreover, the cathode holder is heated up to a high temperature and thermal electrons are emitted therefrom. This may cause unwanted electric discharge (arc discharge) between the cathode holder and the plasma generating chamber thus causing a loss as well as contaminating the inside of the plasma generating chamber.